A well-known type of optical spectrometer uses a grating to diffract light. A grating is, for example, an array of fine, substantially parallel, substantially equally spaced grooves (“lines” or “rulings”) on a reflecting or transparent substrate. The grooves result in diffractive mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions for which constructive interference occurs. These directions are wavelength dependent, except for the direction of straight transmission or reflection (angle of incidence=angle of reflection), which is not considered to be included in the definition of “diffraction” as used herein. In a spectrometer, the resulting wavelength-dependent dispersion is used to analyze the spectral properties of the light. An intensity-detection device (e.g., a photo-detector) measures the intensity of the light as a function of angle of diffraction.
For applications where a sensitive measurement of the spectral properties of weak incident light is required, it is desirable that such a spectrometer has a high-diffraction efficiency for the diffraction order or orders that are used. Such a desire exists, for example, for applications like Raman Spectroscopy, in which the spectrum has to be analyzed of generally low-intensity light that has been inelastically scattered by an object under investigation.
Unfortunately, under certain circumstances, the efficiency of diffraction from a grating is not very high, or only high for one polarization direction of the incident light, which can be the case, e.g., for gratings used under grazing incidence in which such effects already become noticeable when the angle between the direction of the incoming light and the normal of the plane of the grating surface increases to values above 60 degrees. Usually, different polarizations of light are also diffracted optimally in different wavelength regions.